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Chapter-5
MODELING OF UNIFIED POWER FLOW CONTROLLER
5.1 Introduction
There are a number of FACTS devices that control power system
parameters to utilize the existing power system and also to enhance
the dynamic performance and stability of the power system. Out of
these, UPFC is the most flexible multi functional FACTS device. UPFC
perform the functions of a shunt reactive current injection to control
bus voltage and inject series reactive voltage to control power flow in
transmission line [23][30][33]. If PI Controllers equipped by the UPFC
shunt and series controllers are slow or if PI controllers are not
properly tuned or if the UPFC operates manually, the UPFC is not in a
position to effectively damp the power system oscillations [28]. To
achieve this, power oscillation damping control stability loop or
auxiliary controller is added along with power flow controller [23].
5.2 Unified Power Flow Controller (UPFC)
5.2.1 Configuration
The basic concept diagram of UPFC is shown in Fig.5.1. It
contains two back to back AC to DC synchronous voltage sourced
converters (VSC1 and VSC2) operated with common DC link capacitor
[23] [28] [29]. VSC1 is connected in shunt through shunt-connected
transformer and VSC2 is connected in series through series connected
transformer. The shunt branch of UPFC comprised of a DC Capacitor,
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VSC1 and a shunt-connected transformer corresponds to a
STATCOM. It can absorb or generate only reactive power because the
output current is in quadrature with the terminal voltage. The series
branch of UPFC is comprised of a DC Capacitor, VSC2 and a series
connected transformer corresponds to a SSSC. It can act as a voltage
source injected in series to the transmission line through series
connected transformer; the current flowing through the VSC2 is the
transmission line current (I) and it is function of the transmitted
electric power and the impedance of the line. The injected voltage (Vse)
is in quadrature with the transmission line current (I) with the
magnitude being controlled independently of the line current. Hence,
the two branches of the UPFC can absorb or generate the reactive
power independent of each other.
If the two converters (VSC1 and VSC2) are operating at the
same time, the shunt and series branches of the UPFC can basically
function as an ideal ac to ac converter in which the real power can
flow in either direction through the dc link and between the AC
terminals of the two converters. The real power from VSC1 to VSC2
and vice versa, and hence it is possible to introduce positive or
negative phase shifts between V1 and V2. The series injected voltage
Vse can have any phase shift with respect to the terminal voltage V1.
Therefore, the operating area of the UPFC becomes the circle limited
with a radius defined by the maximum magnitude of Vse, i.e.,
Vse.max.
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The VCS2 is used to generate the voltage Vse 0 ≤ Vse ≤ Vse.max
and phase shift 0 ≤ θ ≤ 2π at the fundamental frequency. This voltage
is added in series to the transmission line and directly to terminal
voltage V1 by the series connected coupling transformer. The
transmission line current passes through the series transformer, and
in the process exchanges real and reactive power with the VSC2. This
implies that the VSC2 has to be able to absorb and deliver both real
and reactive power.
The shunt-connected branch associated with VSC1 is used
primarily to provide the real power demanded by VSC2 through the
common DC link terminal. Also, it can generate or absorb reactive
power independently of the real power, it can be used to regulate the
terminal voltage V1; thus, VSC1 regulates the voltage at the input
terminals of the UPFC.
Another important role of the shunt branch of UPFC is a direct
control of the DC capacitor voltage, and consequently an indirect
regulation of the real power required by the series UPFC branch. The
amount of real power required by the series converter plus the circuit
losses have to be supplied by the shunt converter. Real power flow
from the series converter to shunt converter is possible and in some
cases desired, in this case, the series converter would supply the
required real power plus the losses to the shunt converter.
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Fig.5.1 The basic scheme of UPFC
5.2.2 UPFC transmission control capabilities
The power transmission with UPFC based on the reactive shunt
compensation, the series compensation and the phase angle
regulation. The UPFC can meet multiple control objectives by adding
the series injected voltage with appropriate magnitude and phase
angle to the terminal voltage V1. Using phasor representation, the
basic UPFC power flow control functions are illustrated in Fig.5.2.
Voltage regulation with continuously variable in phase / anti
phase voltage injection is shown in Fig.5.2 (a) for voltage
increments Vse = ±ΔV (σ =0).
Series reactive compensation is shown in Fig.5.2 (b), where Vse
= Vq is in quadrature with line current I. functionally this is
similar to series capacitive and inductive line compensation by
the SSSC.
Phase angle regulation is shown in Fig.5.2(c), where Vse = Vσ is
injected with an angular relationship with respect to Vs that
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archive the function as a perfect phase angle regulator, which
can also supply the reactive power involved with transmission
angle control by internal VAR generation.
Multifunction power flow control executed by simultaneous
terminal voltage regulation, series capacitive line compensation
and phase shifting is shown in Fig.5.2 (d) where Vse = ΔV + Vq +
Vσ. This functional capability is unique to the UPFC. No single
conventional equipment has similar multi functional capability.
Fig.5.2 Phasor representation of UPFC power flow control
functions
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5.2.3 UPFC control system
The general control scheme of UPFC [31] [33] is as shown in
Fig.5.3. The UPFC is a multi variable control device with four inputs
(magnitude and phase angle of the shunt and series converter output
voltages) and four outputs (real and reactive output powers of the
shunt and series converters). The series converter controls the active
and reactive powers flow through transmission line by adjusting the
magnitude and phase angle of the series injected voltage. The shunt
converter controls the dc voltage and the bus voltage (V1) at the shunt
converter transformer. In this thesis, the shunt converter is used to
control the sending-end bus voltage magnitude by locally generating
and absorbing reactive power. The series converter directly controls
real line power by the magnitude of the series injected voltage.
Fig.5.3 Basic control structure of UPFC
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5.2.3.1 Shunt converter controls
The shunt converter has two duties, namely, to control the
voltage magnitude at the sending-end bus (Bus V1 in Fig.5.1) by
locally generating or absorbing reactive power, and to supply or
absorb real power at the dc terminals as demanded by the series
converter. It is possible to achieve real power balance between the
series and shunt converter by directly controlling the dc voltage Vdc,
as any excess or deficit of real power will tend to increase or decrease
the dc voltage, respectively.
By varying the magnitude and angle of the shunt converter
output voltage the real and reactive power flow in and out of the shunt
converter is controlled [23][31]. The PI bus voltage regulator as shown
in Fig.5.4 (a) sets the reactive current reference and PI dc voltage
regulator sets real current reference as shown in Fig.5.4 (b) This
control scheme is basically the same as a STATCOM control.
The d–q decoupled current control strategy for shunt converter
[45] is implemented as shown in Fig.5.3.
The control system consists of:
A phase-locked loop (PLL): it is used to synchronize the Shunt
converter current with sending-end bus voltage (V1) at the
point of UPFC connection.
An AC voltage regulator (Bus-voltage regulator): it gives the
reference reactive current Iqref required by the system to
maintain bus voltage at constant value or in specified range.
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A DC voltage regulator: it gives the reference active current
Idref to maintain the capacitor voltage at a constant value or in
specified range.
The inner current regulator: it controls the magnitude and
phase of the voltage generated by the PWM converter of Shunt
converter to deliver or absorb required reactive current by the
Shunt converter as per reference valve given by the AC and Dc
voltage regulators.
Fig.5.4 Shunt converter current controller
The shunt converter controls the bus voltage by injecting
reactive current in quadrature with sending-end voltage V1. The
magnitude of the shunt voltage can be calculated by the following
equation
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Vsh = Vref + XS.I ---------------5.1
Where
Vsh = Positive sequence voltage (pu) of shunt converter
I = Reactive current (pu/Pnom)
XS = Slope (pu/Pnom: usually between 1% and 5%) or the leakage
reactance of shunt connected transformer and series reactance
connected between converter and power system
The voltage Vsh is controlled through the changes in the
amplitude modulation ratio msh, as the output voltage magnitude is
directly proportional to msh according to the following equation
Vsh = (1/2√2)*msh*Vdc -------------5.2
5.2.3.2 Series converter controls
Two different control schemes for the series converter were
implemented. One scheme to control real power flow through
transmission line and voltage magnitude at the receiving-end bus;
another control scheme for controlling the real and the reactive power
flows through the transmission line.
From the basic principle of UPFC, series converter does main
function of UPFC. The series converter active and reactive powers are
controlled by using two separate PI controllers, taking advantage of
the UPFC ability to independently control reactive and real power. The
basic principle of real power flow being directly affected by changes in
phase angles, while reactive power flow is directly associated with
voltage magnitudes, is used here to design the UPFC control.
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The outputs of the PI controllers are d and q components of the
series injected voltage Vse, i.e., Vsed and Vseq respectively. The
magnitude of the series voltage can be calculated by the following
equation
Vse-q = (Kp + Ki/S)*(Pref - P) ------------ (5.3)
Vse-d = (Kp + Ki/S)*(Qref - Q) ------------ (5.4)
Vse = √ (Vse-d2 + Vse-q
2) ------------ (5.5)
The amplitude modulation ratio
mse = √ (8*Vse/Vdc) ---------------------(5.6)
The phase angle of the series injected voltage with respect to the
reference waveform, i.e., the sending-end voltage V1 is given as
follows
β = -tan-1 (Vse-q/Vse-d)
Fig.5.5 Series converter injected voltage controller
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The series converter controls active power flow in line by
controlling the magnitude of the series injected voltage, injecting in
qudrature with the line current I.
5.3 SIMULINK Modeling of UPFC
The SIMULINK model of UPFC developed as a phasor model, to
perform dynamic and transient stability studies in 3-Ph power
systems. The series converter (VSC2) injected voltage (Vq) is controlled
to meet the power demand in the line set by the reference power set
point (Pref) and shunt converter (VSC1) delivers or absorbs the reactive
current as per the output of ac voltage regulator.
5.3.1 PI Voltage Controller of shunt converter
The SIMULINK model of PI voltage controller block diagram for
UPFC shunt converter is shown in Fig.5.6. This controller gives
appropriate shunt reactive current injected into the power system at
which UPFC located for appropriate change in bus voltage with
respect to the reference voltage.
Fig.5.6 PI Voltage Controller block diagram of UPFC shunt Converter
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5.3.2 FLPOD controller along with PI Voltage Controller of shunt converter
The SIMULINK model of FLPOD controller along with PI Voltage
Controller block diagram for Shunt Current Controller of UPFC is
shown in Fig.5.8. FLPOD shunt controller is fed by one input namely
change in power or difference in power (DP) of a constant resistive
load connected parallel to the shunt converter to the UPFC. This gives
the appropriate shunt current (Iq), which is required by the system
during transient period and it gives zero output for steady state.
The rules for the proposed FLPOD shunt controller are:
i) If „DP‟ is „DPN‟ (DP Negative) Then „Iq‟ is „IqN‟ (Iq Negative)
ii) If „DP‟ is „DPZ‟ (DP Zero) Then „Iq‟ is „IqZ‟ (Iq Zero)
iii) If „DP‟ is „DPP‟ (DP Positive) Then „Iq‟ is „IqP‟ (Iq Positive)
These rules are in matrix form as given below
error (DP)
Out put (IQ)
DPN IQN
DPZ IQZ
DPP IQP
The membership functions for input and output of FLPOD
shunt controller, Change in power or difference in power (DP) and
shunt injected current (Iq) are given in Fig.5.7.
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Fig.5.7 (a) Input membership function (DP) and (b) Output Membership function (Iq) of FLPOD shunt controller
Fig.5.8 FLPOD controller along with PI Voltage Controller block diagram of UPFC shunt converter
5.3.3 PI Power Flow Controller of series converter
The SIMULINK model for PI Power Flow controller of series
converter block diagram is shown in Fig.5.9. This controller gives
appropriate series injected voltage for appropriate change in line
power with respect to the reference power.
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Fig.5.9 PI power flow controller block diagram of UPFC series converter
5.3.4 FLPOD controller along with PI Power flow controller of series converter
The SIMULINK model for Series voltage controller of UPFC with
FLPOD controller along with PI power flow controller is shown in
Fig.5.11. FLPOD controller is fed by one input namely change in
power or difference in power (DP). This gives the appropriate series
injected voltage (Vq), which is required by the system during
transients and it gives zero output under steady state.
The rules for the proposed FLPOD series controller are:
i) If „DP‟ is „DPN‟ (DP Negative) Then „Vq‟ is „VqN‟ (Vq Negative)
ii) If „DP‟ is „DPZ‟ (DP Zero) Then „Vq‟ is „VqZ‟ (Vq Zero)
iii) If „DP‟ is „DPP‟ (DP Positive) Then „Vq‟ is „VqP‟ (Vq Positive)
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These rules are in matrix form as given below
error (DP)
Out put (Vq)
DPN VqN
DPZ VqZ
DPP VqP
The membership functions for input and output of FLPOD
controller, Change in power or difference in power (DP) and series
injected voltage (Vq) are given in Fig.5.10 (a and b)
Fig.5.10 (a) Input membership function (DP) and (b) Output
Membership function (Vq) of FLPOD series controller
Fig.5.11 FLPOD controller along with PI power flow controller block diagram of UPFC series converter
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5.4 Summary
In this chapter details of UPFC have been discussed. SIMULINK
implementation of the UPFC has been discussed. The UPFC with PI
and FLPOD controllers allows the controls of the amplitude of both
shunt reactive current and series injected reactive voltages.